Vaccines are one of the greatest achievements of biomedical science and public health. At the beginning of the 20th century, infectious diseases were widely prevalent in the United States and exacted an enormous toll on the population. For example, in 1900, 21,064 smallpox cases were reported, and 894 patients died. In 1920, 469,924 measles cases were reported, and 7575 patients died; 147,991 diphtheria cases were reported, and 13,170 patients died. In 1922, 107,473 pertussis cases were reported, and 5099 patients died. These diseases have largely been eliminated in the United States.
Despite this success, more than 5 million infants world-wide die every year from diseases that could be avoided with existing vaccines. However, many of the current vaccines must be refrigerated which makes their distribution in developing countries difficult. Furthermore, vaccines are either nonexistent or not available for diseases associated with significant rates of morbidity or mortality. For example, more than 250 million people are chronically infected with hepatitis B virus, malaria causes 1–2 million deaths each year; diarrheal diseases (for example, infections caused by rotovirus, Shigella sp., Vibrio cholera, and toxin producing E. coli) annually kill more an estimated 4–5 million people.
The Center for Disease Control has identified several factors for achieving the full potential of vaccines (www.cdc.gov/epo/mmwr/preview/mmwrhtml/00056803.htm). These suggestions include pursuing new approaches to vaccine delivery and administration. One new approach is the development of vaccines that stimulate the two types of immune responses: humoral responses mediated by B cells and cellular responses mediated by helper T-cells.
However, attempts to create vaccines that stimulate both humoral and cellular immune responses or preferentially induce a cellular immune response have met with difficulty. For example, synthetic peptide vaccines and recombinant protein vaccines are often poorly immunogenic and tend to induce humoral responses and not to induce cellular immune responses. DNA vaccines can induce both humoral and cellular immune responses. However, questions remain as to what the consequences of long-term antigen expression will be.
Accordingly, what is needed in the art are improved delivery mechanisms for vaccines. In particular, the delivery mechanism should be useful inducing both cellular and humoral immune responses.